Microwave Plasma Spectrometer Using Dielectric Resonator

ABSTRACT

A dielectric resonator is excited at its natural resonant frequency to produce a highly uniform electric field for the generation of plasma. The plasma may be used as a desolvator, atomizer excitation source and ionization source in an optical spectrometer or a mass spectrometer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is divisional application based on U.S. applicationSer. No. 14/775,497 filed Sep. 11, 2015, being a national stageapplication of international application PCT/US2014/024312 having aninternational filing date of Mar. 12, 2014 and claiming the benefit ofU.S. provisional application 61/779,557 filed Mar. 13, 2013 all herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to electrical antennas and inparticular to an antenna generating an efficient and uniformelectromagnetic field for plasma generation and the like. The presentinvention is particularly directed to the generation of plasma intowhich is introduced a substance to be analysed, the plasma causingatomization, excitation and ionization of the substance so that thelight it then emits or absorbs, or the ions produced, may be analysed todetermine properties of the substance.

BACKGROUND TO THE INVENTION

Plasma sources are used for the generation of gaseous plasma whoseunique physical, chemical, optical, thermal, and biological effects areextensively used in broad areas of science and industry. High-frequencyplasma sources utilize radio-frequency or microwave electrical energy tosustain a plasma. High-frequency plasma sources typically include aradio frequency (RF) shield in order to minimize human exposure to highintensity non-ionizing radiation and reduce electromagnetic interferenceand power losses due to the radiation of electromagnetic energy.Although plasma is typically produced inside an RF shielded enclosure,the beneficial effects of plasma may be realized either inside oroutside the radio frequency shield.

Plasma sources which use radio-frequency or microwave energy to sustainplasma are usually classified as belonging to one of two broadcategories, capacitively coupled or inductively coupled. A capacitivelycoupled plasma source relies on electrical charges stored on capacitorplates to produce an electric field which accelerates the electrons andions in the plasma. On the other hand, an inductively coupled plasmasource relies on a changing magnetic field, produced by the currentflowing through a coil, to induce an electric field in the plasma asdescribed by the Faraday's law of induction. Both capacitively andinductively coupled plasma sources find extensive application in theprocessing of semiconductor wafers. While the capacitively coupledsources are suitable for producing a uniform low-pressure plasma over arelatively large area, inductively coupled sources are capable ofproducing higher density plasma within a smaller volume. In addition,inductive sources are more efficient in coupling large amounts ofelectrical power into highly electrically conductive plasma, such as inatmospheric plasma torches which generate very high temperature plasmaat atmospheric pressure with many applications in science and industry.The present invention relates to inductively coupled plasma sources.High frequency electrical fields for the generation of plasma may makeuse of a conductive coil (“field applicator”) driven by an AC currentoscillating in the MegaHertz to GigaHertz range. A gas within the coilreceives energy from the coil through inductive coupling exciting thegas into a plasma state.

Such inductive coupling techniques for generating plasma have a numberof significant problems. First, normally the conductive coil must havemultiple “turns” and each turn exhibits a mutual capacitance withadjacent turns of the loop creating field (and hence plasma)inhomogenieties which may be manifested as nonuniform plasma ion speeds,trajectories and densities.

Nonuniformities in the plasma may adversely affect applications in whicha uniform plasma is required (for example, for etching in the integratedcircuit industry) and may waste energy on undesired plasma processes.Since the regions of plasma with higher electron density absorb morepower than the regions with lower electron density, the ionization isfurther enhanced in high density regions and reduced in low densityregions, which may lead to instability. The less uniform the electricfield, the more likely it is that the plasma will exhibit instabilitiesranging from a departure from a local thermodynamic equilibrium to acontraction into a filamentary discharge. Furthermore, a disproportionalenergy absorption by the plasma in the regions of high field intensity,which are usually located close to the antenna, limits the energyavailable to other regions of the plasma. The mutual capacitance alsolimits the voltage that may be applied to the conductive coil withoutdielectric breakdown between the turns of the coil.

Second, the large amount of electrical power and hence large amounts ofelectrical current required to pass through the conductive coil producesignificant resistive heating requiring complicated or bulky coolingstructures. The use of highly conductive materials, such as copper, canreduce resistive losses, but the use of copper and similar metals iscomplicated by the susceptibility of such highly conductive materials tocorrosion and melting in the harsh environment of the plasma.

Third, efficient driving of the conductive loop requires that the loopbe part of a resonant structure implemented by placing a tuningcapacitor into the coil circuit.

Capacitors suitable for this purpose are expensive and bulky, and thetuning capacitor may require automated control in order to match thediffering load when firstly igniting the plasma and then after stableplasma has been formed, adding further cost and complexity.

SUMMARY OF THE INVENTION

In first embodiment the present invention provides an optical emissionspectrometer or a mass spectrometer comprising a plasma generator, theplasma generator comprising a dielectric resonator structure having acentral axis and a radiofrequency power source electrically coupled tothe dielectric resonator structure to promote an alternatingpolarization current flow at a natural resonant frequency of thedielectric resonator structure about the axis to generate plasma in anadjacent gas.

A further embodiment the present invention provides a method ofanalyzing a substance comprising the steps of: generating plasma using aplasma generator including a dielectric resonator structure and aradiofrequency power source electrically coupled to the dielectricresonator structure to promote an alternating polarization current flowat a natural resonant frequency of the dielectric resonator structureabout the axis to generate plasma in an adjacent gas; introducing a gasinto a region adjacent to the dielectric resonator structure; excitingthe dielectric resonator structure at a natural resonant frequency togenerate plasma in the introduced gas; introducing substance to beanalyzed into the plasma; dispersing light emitted by the substanceaccording to the wavelengths of the light or separating ions of thesubstance created by the plasma according to their mass to charge ratio;detecting either light emitted by the substance according to thewavelengths of the light or ions of the substance created by the plasmaaccording to their mass to charge ratio; and determining the elementalcomposition of the substance either from the wavelengths of lightdetected or from the mass to charge ratio of the ions detected.

The radiofrequency power source is preferably electromagneticallycoupled to the dielectric resonator structure. The dielectric resonatorstructure is preferably electrically coupled to the plasma substantiallyonly by induction, there being negligible capacitive coupling.

Preferably the dielectric resonator has a quality factor greater than100. Preferably the dielectric resonator has electrical resistivitygreater than 1×10¹⁰ Ω·cm. Preferably the dielectric resonator has amelting point greater than a melting point of copper. Preferably thedielectric resonator has a loss tangent of less than 0.01. Preferablythe dielectric resonator has a dielectric constant greater than five.Preferably the dielectric resonator is selected from the groupconsisting of alumina (Al₂O₃) and calcium titanate (CaTiO₃). Preferablythe dielectric resonator is a ring or cylindrical annulus having acentral opening along the axis. Preferably the ring or cylindricalannulus has a central opening which is circular and has a diameter ofbetween 15 mm and 25 mm.

Preferably the adjacent gas comprises nitrogen or air.

Preferably the radiofrequency power source provides between 0.5 and 2 kWof power which is able to be coupled into the plasma. Preferably theradiofrequency power source is driven at a frequency which is within twofull width at half maximum (FWHM) bandwidths of the resonant frequencyof the dielectric resonator structure when the resonator is loaded.Preferably the radiofrequency power source automatically seeks thenatural resonant frequency of the dielectric resonator structure tooutput radiofrequency power at or substantially at the natural resonantfrequency of the dielectric resonator structure.

The present invention provides an antenna structure for generatingplasma by using a dielectric antenna. The present inventors havedetermined that such antennas when fabricated with the material havinghigh dielectric constant and low dielectric losses can be operated atresonance to provide for high field strengths with low powerdissipation.

While the inventors do not wish to be bound by a particular theory, itis understood that the invention replaces the “conduction” current ofelectrons in a conventional coil with a “polarization” current ofelectrons in the dielectric material. The polarization current is due tothe minor displacement of elementary charges bound to molecules of thedielectric material under the influence of an electric field. Both typesof current (conduction current and polarization current) produce amagnetic field and an induced electric field according to the same lawsof electromagnetism. However, since the dielectric material is at onceits own capacitor and an inductor, the electric-potential is exactlyzero everywhere inside the dielectric and in the space around thedielectric.

As there are neither free nor bound charges, at a macroscopic level theelectric potential is exactly zero within and around the dielectric, andthe electric field is produced purely by induction, due to the rate ofchange of the magnetic vector potential in accordance with equation (1):

$\begin{matrix}{\overset{\rightarrow}{E} - {\nabla V} - \frac{\partial\overset{\rightarrow}{A}}{\partial t}} & (1)\end{matrix}$

where

Ē—vector of electric field strength

∇—gradient operator

V—electric scalar potential (or simply electric potential, potential, orvoltage)

Ā—magnetic vector potential (or simply vector potential).

Equation (1) may be found in standard texts on electromagnetism, such asequation 6.31 on page 179 of “Classical Electrodynamics” by J. D.Jackson, John Willey & Sons, 1962.

The ∇V component of the electric field is sometimes referred to as theelectrostatic component and the ∂Ā/∂t component is sometimes referred toas the induced component.

The second term in the right hand side of equation (1) is due to theFaraday's law of induction and may exist even when V=0 everywhere. In aconventional inductively-coupled-plasma (ICP) coil, Ā≠0 due to thecurrent flowing through the coil, and V≠0 due to a large voltagedifference between the ends of the coil or, rather, due to theelectrical charges stored on the surface of the coil. However, in anaxially symmetric dielectric resonator as used in the present invention,Ā≠0 due to the polarization current, but V=0 because there are neitherfree nor bound charges.

Parasitic capacitive coupling is therefore entirely eliminated and theelectric field is produced solely by induction. It is further believedthat improved current distribution is obtained through lack of “skin”effects in the dielectric material that cause conductive current flow,unlike polarization current, to concentrate in the outermost portions ofa ring structure. The polarization current density is nearly uniformacross the cross section of the dielectric in much the same way that anelectric field is uniformly distributed across the cross-section ofdielectric in a capacitor. The skin effect or the rapid attenuation ofelectromagnetic waves as they penetrate into a conducting material iseffectively absent in low-loss dielectric materials.

The elimination of capacitive coupling is a considerable advantage overICP sources which suffer particularly large capacitive coupling due totheir use of a multi-turn coil. A conventional method of reducing theparasitic capacitive coupling is to interpose an electrostatic orFaraday shield between the coil and the plasma. Since a solid conductivesheet would block both the inductive and the capacitive components ofthe electric field, the electrostatic shield usually has a series ofnarrow slots normal to the direction of the current in the coil. Adisadvantage of an electrostatic shield is that it reduces the inductivecoupling between the coil and the plasma, for several reasons: a) thecoil must be placed further away from the plasma in order to accommodatethe electrostatic shield, b) screening currents, opposite of the antennacurrent, flow along the portion of the shield which does not have slots,c) the vicinity of the shield to the coil adds significant capacitiveloading which increases the current and Ohmic losses in the coil. Inaddition, the small spacing between conductors limits the maximum powerdue to the reduced breakdown voltage. Finally, the deviation of theelectric field from an ideal inductive field is the largest in thevicinity of the slots where the coupling to the plasma is mostsignificant.

In addition to parasitic capacitive coupling and the limitations imposedby the electrostatic shields, the conventional inductively plasmasources suffer from the following limitations:

a) Large currents in coil conductors dissipate significant amount ofheat which must be removed by fluid cooling, requiring a fluid manifoldand a chiller. Use of dielectric cooling fluids which are damaging tothe environment is not uncommon in semiconductor applications. Addedcomplexity, size, and cost of the cooling system make the conventionalinductively coupled plasma sources unsuitable for design scaling,portable applications, and designs where space available for the plasmasource is limited.

b) The corrosion which builds on the surface of the coil over a periodof time greatly increases Ohmic losses in the coil and may necessitate acoil replacement.

c) Coils made of metal, such as Copper, melt at relatively lowtemperature, are degraded by plasma sputtering, and are incompatiblewith ultra-high-vacuum processes. Therefore, in low-pressure plasmaapplications, the coil must be separated from the plasma by the walls ofthe vacuum chamber and in atmospheric pressure plasma applications, thecoil must be located at a sufficient distance from the plasma. Thisreduces the inductive coupling between the coil and the plasma andcomplicates the mechanical construction of the plasma source.

d) The difference of electric-potential between the turns of the coiland the coil and the shield may cause a dielectric breakdown, limitingthe maximum power that can be processed.

e) The inductance of the coil must be resonated with a tuning capacitor,typically a bulky and expensive variable vacuum capacitor forming a partof an external impedance matching network, adding to the size, cost, andcomplexity of the plasma source, while further limiting the maximumpower that can be processed and reducing the efficiency due to thelosses in the impedance matching network.

The present invention advantageously avoids all these problems,providing improved plasma uniformity, better control of ion speeds andtrajectories, reduced deposition or sputtering of the walls of anyplasma chamber, better efficiency in coupling electrical energy intouseful plasma processes, higher limits to the power that can be coupledinto useful plasma processes and complete elimination of theelectrostatic or Faraday shield.

Specifically then, the present invention provides a plasma generatorhaving a dielectric resonator structure having a central axis and aradiofrequency power source electrically coupled to the dielectricresonator structure to promote an alternating polarization current flowat a natural resonant frequency of the dielectric resonator structureabout the axis to generate plasma in an adjacent gas. The radiofrequencypower source is electrically coupled to the dielectric resonator. As amagnetic field is also present, the radiofrequency power source is bothelectrically coupled and magnetically coupled to dielectric resonatorstructure; hence the radiofrequency power source may be said to beelectromagnetically coupled to the dielectric resonator structure. Thecoupling promotes an alternating polarization current flow at a naturalresonant frequency of the dielectric resonator structure. Theradiofrequency power source is driven at a frequency or a range offrequencies (such as broadband) which is sufficient to couple at leastsome power into the dielectric resonator structure at its naturalresonant frequency. Preferably the radiofrequency power source is drivenat a frequency which is related to the natural resonant frequency of thedielectric resonator structure. More preferably the radiofrequency powersource is driven at a frequency which is within two full width at halfmaximum (FWHM) bandwidths of the resonant frequency of the dielectricresonator structure when the resonator is loaded. The bandwidth of anunloaded dielectric resonator is very narrow and may broaden by a factorof 100 when loaded with the plasma.

It is thus a feature of at least one embodiment of the invention toprovide an improved radiofrequency antenna for the generation of intensebut uniform electrical fields for plasma production.

The dielectric resonator may have any one or more of the qualities of: aquality factor of greater than 100, an electrical resistivity greaterthan 1×10¹⁰ Ω·cm, a dielectric constant with a loss tangent of less than0.01, and a dielectric constant greater than five.

It is thus a feature of at least one embodiment of the invention toprovide a dielectric material that produces extremely low losses atradiofrequency fields and high power levels to minimize problems ofcooling and energy loss.

The dielectric resonator may be of a material having melting pointgreater than a melting point of copper.

It is thus a feature of at least one embodiment of the invention toprovide a material that is robust against the extremely hightemperatures of plasma.

The dielectric material may, for example, be alumina (Al₂O₃) or calciumtitanate (CaTiO3).

It is thus a feature of at least one embodiment of the invention toprovide an apparatus that may be constructed of relatively common andmanufacturable materials.

The dielectric resonator may be a ring having a central opening alongthe axis.

It is thus a feature of at least one embodiment of the invention toprovide a dielectric resonator that is relatively simple to manufacture.

The ring may have a central opening of at least one millimeter diameteror at least one half inch. The ring may have a central opening whichdiffers according to the area of application of use of the dielectricresonator. The central opening may be circular in it may be any othershape convenient to the application. Preferably the central opening iscircular. Where the central opening is circular it will have acharacteristic dimension which is its diameter. Where the centralopening is not circular the size of the central opening will have one ormore characteristic dimensions which are representative of widths acrossthe opening. For use in the fields of optical spectroscopy and massspectrometry the central opening may have a characteristic dimensionbetween 1 mm and 50 mm. For use in the field of lasers the centralopening may have a characteristic dimension between 1 mm and 1 m. Foruse in the fields of electron cyclotron resonance plasma sources thecentral opening may have a characteristic dimension between 10 mm and500 mm. For use in the field of semiconductor processing the centralopening may have a characteristic dimension between 10 mm and 1 m. Foruse in the fields of material processing and propulsion the centralopening may have a characteristic dimension between 1 mm and 1 m. Foruse in the field of ICR heating the central opening may have acharacteristic dimension between 1 m and 20 m.

Preferably, for use in the fields of optical spectroscopy and massspectrometry the central opening is circular and has a diameter ofbetween 1 mm and 50 mm, more preferably between 5 mm and 30 mm, morepreferably still between 15 mm and 25 mm.

The dielectric resonator may take the form of a cylindrical annulus,having a central opening concentric with the outer diameter of theannulus. However other shapes of dielectric resonator are contemplated.Preferably the dielectric resonator takes the form of a cylindricalannulus, having a central opening concentric with the outer diameter ofthe cylindrical annulus.

It is thus a feature of at least one embodiment of the invention toprovide a dielectric resonator that is readily adaptable to formingplasma in flowing gas.

To that end, the plasma generator may include a gas port introducing gasinto the ring along an axis of the ring.

It is thus a feature of at least one embodiment of the invention toprovide the elements of a plasma torch for spectroscopic or otherapplications.

The radiofrequency power source may automatically seek the naturalresonant frequency of the dielectric resonator structure to outputradiofrequency power at the natural resonant frequency of the dielectricresonator structure. This may be readily achieved by creating a phaselock between the amplifier signal and the wave reflected from theresonator using a directional coupler as a detector.

It is thus a feature of at least one embodiment of the invention toprovide a plasma generator that may automatically adjust to variationsin the dielectric resonant material or its environment. Such variationsinclude changes caused by altered plasma conditions, such as the changeof plasma gas, pressure, sample type (aqueous or organic), and gas andsample flow rates when the invention is applied to the fields of opticalspectroscopy and mass spectrometry, for example. In addition, thepermittivity of low-loss dielectric materials and the dimensions ofexternal components in the environment of the dielectric oscillator,such as an RF shield, may change with temperature which may affect thetuning in applications which require extreme operating temperatures,such as a microwave rocket nozzle.

Preferably the radiofrequency power source automatically seeks thenatural resonant frequency of the dielectric resonator structure tooutput radiofrequency power at or substantially at the natural resonantfrequency of the dielectric resonator structure.

The radiofrequency power source may be a magnetron or a solid-state orvacuum tube oscillator. The radiofrequency power source may comprise oneor more of a magnetron, a solid state oscillator or a vacuum tubeoscillator.

Preferably the dielectric resonator structure is electrically coupled tothe plasma substantially only by induction, there being negligiblecapacitive coupling.

It is thus a feature of at least one embodiment of the invention topermit the generation of extremely high frequency plasma. The inventionmay be utilized with radiofrequency power sources operating at leastwithin the range of 1 MHz to 10 GHz, and specifically within the VHFrange (30 MHz-300 MHz) and the UHF range (300 MHz-3 GHz).

Plasmas may be sustained in a variety of gases, including but notlimited to argon, nitrogen, helium and air. Plasmas may be used in avariety of applications, including high temperature plasma for plasmacutting, welding, melting, and surface treatment of materials,destruction of hazardous materials, vitrification of waste, ignition ofhydro-carbon fuels; light emitted by excited atomic and molecularspecies for optical-emission spectroscopy and light sources; ions formass-spectroscopy, ion-implantation, and ion-thrusters; small particlesfor material spheroidization, synthesis of nano-materials and plasmaspraying of surface coatings; reactive plasma species for gasificationand the production of syngas; supersonic gas flow for scientific andin-space propulsion applications; combination of plasma effects andproducts for lean internal combustion and exhaust detoxification, plasmaassisted combustion, ore reduction and processing, hydro-carbon fuelreforming, air purification and removal of airborne contaminants inresearch facilities, hospitals etc.

The present invention is particularly directed to the excitation andionization of substances so that the light they then emit, or the ionsproduced, may be analysed to determine properties of the substance.Important properties which may be determined include the elementalcomposition of the substance and the relative quantities of elementalcomponents of the substance. The present invention is especiallydirected for application within the fields of optical emissionspectroscopy (OES) and mass spectrometry (MS), the microwave plasmasource replacing, for example, conventional inductively coupled plasma(ICP) sources. Where the plasma is used to excite or ionize a substanceso that it may be analysed using spectroscopy or spectrometry, thesubstance to be analysed is introduced into the plasma. As well asexciting or ionizing the substance to be analysed, the plasma may alsoatomise the substance and it may desolvate the substance. As anatomization source, the plasma generator of the present invention may beused for atomic absorption (AA) spectroscopy.

The optical emission spectrometer of the present invention preferablycomprises an optical sensor, wherein the optical sensor comprises adispersive element for dispersing light emitted by the plasma accordingto the wavelength of the light; and an optical detector for detectingthe dispersed light. Hence the optical emission spectrometer of thepresent invention preferably comprises a plasma generator, the plasmagenerator comprising a radiofrequency power source and a dielectricresonator; a dispersive element for dispersing light emitted by theplasma according to the wavelength of the light; and an optical detectorfor detecting the dispersed light. Preferably the optical emissionspectrometer will further comprise one or more of: one or more opticalfocusing elements which may be lenses or mirrors; mirrors for changingthe direction of one or more beams of light; a focal plane arraydetector comprising multiple detecting elements for simultaneouslydetecting light dispersed by the dispersive element, the focal planearray detector forming at least part of the optical detector; acontroller for controlling the spectrometer; and a controller forreceiving an output from the optical detector, which may be the samecontroller as is used for controlling the spectrometer. In a preferredform, the dispersive element comprises a grating.

The mass spectrometer of the present invention preferably comprises agas port suitable for delivering sample material into the plasmagenerated by the plasma generator; a sample cone and a skimmer cone; atleast one ion focusing element; a mass analyzing element; and an iondetector for detecting sample material ionized by the plasma. Preferablythe mass spectrometer further comprises a controller for controlling themass spectrometer and a controller for receiving an output from the iondetector.

The particular objects and advantages described may apply to only someembodiments falling within the claims and thus do not define the scopeof the invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cutaway perspective view of a plasma generator usinga ring dielectric resonator of one embodiment of the present invention;

FIG. 2 is a top plan view of the ring dielectric resonator of FIG. 1showing the orientation of polarization current flow;

FIG. 3 is a model showing the electrical field in the ring dielectricresonator of

FIG. 4 is a perspective view of an alternative embodiment of a ringdielectric resonator having standoffs for thermal conduction path to asupporting structure and airflow;

FIG. 5 is a perspective view of a ring dielectric resonator fabricatedof individual sectors and showing one such sector;

FIG. 6 is a perspective view of a dielectric resonator fabricated frommultiple laminated rings;

FIG. 7 is a perspective partial cutaway view of a dielectric resonatorfabricated from a rod having circumferential grooves and a central axialbore;

FIG. 8 is a perspective partial cutaway view of a disk dielectricresonator showing an external plasma region;

FIG. 9 is a perspective partial cutaway view of a disk dielectricresonator providing a stepped surface disk to produce an axialdisk-shaped plasma;

FIG. 10 is a perspective partial cutaway view of a nozzle for use inplasma cutting and welding or plasma thrusters;

FIG. 11 is a fragmentary view of a loop power coupling system similar tothat shown in FIG. 1 for inductively coupling electrical power into thedielectric resonator;

FIG. 12 is a figure similar to FIG. 11 showing a coupling systememploying a microwave waveguide;

FIG. 13 is a perspective view of two identical ring-shaped dielectricresonators movable with respect to each other for tuning;

FIG. 14 is a figure similar to FIG. 13 showing alternative tuningstructure in which one dielectric resonator may fit over the otherdielectric resonator for tuning; and

FIG. 15 is a simplified cross-sectional view of a spectrometerincorporating the dielectric resonator of the present invention.

FIG. 16 is a simplified schematic cross-sectional view of a massspectrometer incorporating the dielectric resonator of the presentinvention.

FIG. 17 shows a plot of signal intensity in counts per second (IR) vs.element concentration for a range of elements present in a test solutionas measured by an optical emission spectrometer of the presentinvention.

FIG. 18 shows a plot of signal intensity in counts per second (IR) vs.element concentration for a range of elements present in a test solutionwhich also contained 3% salt matrix, as measured by an optical emissionspectrometer of the present invention.

FIG. 19 (a) to (d) are peak profile plots showing measured peakintensities from a multielement standard and baselines which arebackground signals from measured blanks (deionised water), for aconventional argon ICP source and the plasma source of the presentinvention operating with air.

FIG. 20 is a perspective partial cutaway view of a dielectric resonatortogether with an RF shield in direct contact with an outer surface ofthe dielectric resonator.

FIG. 21 is a perspective partial cutaway view of a dielectric resonatorin the form of two coaxial ceramic rings, together with two RF shields.

DETAILED DESCRIPTION

Referring now to FIG. 1, a plasma generator 10 of the present inventionmay provide for a dielectric resonator 12 being in this embodiment acylindrical annulus centered about an axis 14.

As is understood in the art, dielectric materials are substantiallyinsulators with respect to direct currents (that is when a dielectric isplaced in an electric field electrical charges do not flow freelythrough the material as they do in a conductor) but can provide forpolarization currents produced by slight shifts in the equilibriumpositions of bound electrons or ions in the material.

In this embodiment, the dielectric resonator 12 may be made of alumina(Al₂O₃) and may be a circular annulus or ring being two inches (0.0508m) in outer diameter, one inch (0.0254 m) in inner diameter and 0.75inches (0.01905 m) in length along axis 14 and having an electricalresonance frequency at approximately 2.45 GigaHertz. This materialexhibits a quality factor of greater than 5000, a relative dielectricconstant of 9.8 and retains its electrical properties and physicalintegrity at temperatures exceeding 1000 degrees centigrade.

An alternative material for the dielectric resonator 12 may be calciumtitanate (CaTiO₃) being 3.13 inches (0.0795 m) in outer diameter 2.34inches (0.05944 m) in inner diameter and 1.12 inches (0.02845 m) inlength and resonating at approximately 430 MegaHertz. This ring exhibitsa quality factor in excess of 5000 and has a relative dielectricconstant of 200.

Many types of advanced technical ceramics meet these requirements, butother dielectric materials with similar electrical properties may beused instead.

More generally, the dielectric material of the dielectric resonator 12may have the following properties: (a) loss tangent less than 0.01, (b)quality factor greater than 100, (c) relative dielectric constant largerthan 5. Alternatively the quality factor should be greater than 1000.

Desirably the dielectric material may have a resistivity greater than1×10¹⁰ Ohm centimeters and typically greater than 1×10¹⁴ Ohmcentimeters. Desirably, the dielectric material may have a melting pointhigher than copper or other comparable conductive metals. The dielectricconstant is preferably greater than five and more desirably greater thannine. These examples are not intended to be limiting. Indeed, dielectricresonators comprising materials with resistivity as low as 100 Ohmcentimeters may be used and there appears to be no practical upper limiton resistivity. Hence the dielectric resonator preferably has electricalresistivity within one of the following ranges: 100-1000 Ohmcentimeters; 1000-10000 Ohm centimeters; 10⁴-10⁵ Ohm centimeters;10⁵-10⁶ Ohm centimeters; 10⁶-10⁷ Ohm centimeters; 10⁷-10⁸ Ohmcentimeters; 10⁸-10⁹ Ohm centimeters; 10⁹-10¹⁰ Ohm centimeters;10¹⁰-10¹² Ohm centimeters; 10¹²-10 ¹⁴ Ohm centimeters; greater than 10¹⁴Ohm centimeters.

The dielectric constant of the dielectric resonator preferably lieswithin one of the following ranges: 5-6, 6-7, 7-8, 8-9 or greater than9.

Preferably the dielectric resonator has a dielectric constant with aloss tangent which lies within one of the following ranges: less than10⁻⁴; 10⁻⁴-10⁻³; 10⁻³-10⁻².

The resonant frequency of a ring is approximately inversely proportionalto the square root of the relative dielectric constant and approximatelyinversely proportional to the linear size of the ring, if all threedimensions of the ring are changed by the same factor, allowing theseexamples to be readily modified to other dimensions.

A precise resonant frequency of a given dielectric resonator may be bestobtained using computer simulations, such as may be achieved usingANSYS-HFSS electromagnetic field solver, for example. However, a firstorder estimate can be obtained by using the following approximateformula which neglects the effect of any RF shield:

$\begin{matrix}{{f_{0} = \frac{c_{0}}{\sqrt{2{\pi ɛ}_{r}{{ht}\left\lbrack {{\ln\left( {8R\sqrt{\frac{\pi}{ht}}} \right)} - 1.75} \right\rbrack}}}},} & (2)\end{matrix}$

where:

c₀=3·10⁸ m/s—speed of light in free-space

ε_(r)—relative permittivity of the dielectric resonator

h—length of the dielectric resonator in [m]

t—thickness of the dielectric resonator, i.e., (O.D.−I.D.)/2 in [m]

R—mean radius of the dielectric resonator, i.e., (O.D.+I.D.)/4 in [m]

Use of equation (2) with a dielectric resonator suitable for use inoptical spectroscopy or mass spectrometry in which the dielectriccomprised a cylindrical annulus of outer diameter 0.0508 m (2″), theresonator having a circular central opening concentric with the outerdiameter of the annulus, the central opening having a diameter of 0.0254m (1″), the dielectric resonator having a thickness (i.e. a cylinderlength) of 0.01905 m (0.75″), and ε_(γ)=9.8, equation (2) provides aresonant frequency of f₀=2.35 GHz. When tested, the measured resonantfrequency was found to be 2.45 GHz, approximately 4% higher than thepredicted value. Hence in practical situations equation (2) may be usedto predict the resonant frequency with a useful accuracy. The dielectricresonator 12 may be positioned near a coupling antenna 16 in turnattached to a radio frequency power supply 18 the latter producing ahigh frequency electrical current exciting the coupling antenna 16 atthe resonant frequency of the dielectric resonator 12. Matching of thefrequency output of the radiofrequency power supply 18 to the resonantfrequency of the dielectric resonator 12 may be done manually byadjusting a frequency setting, or automatically, for example, by using afeedback system detecting impedance changes associated with resonance.Automatic tuning may also be provided by “self resonance” using feedbackfrom a sensing antenna 19 whose output drives the radiofrequency powersupply 18 acting as an amplifier. Self resonance is provided by ensuringa necessary loop phase shift as is generally understood in the art. Byadjusting the phase shift in the loop, such as by changing the length ofthe cable or by using a phase-shifter, one can create the conditions foroscillations. The loop should contain a signal limiting component, suchas a limiter at the input of the amplifier. The radiofrequency powersupply 18 receives electrical power 21, for example, line current from aconventional source.

Referring now to FIG. 1 and FIG. 14, the resonant frequency of thedielectric resonator 12 may be adjusted not only by changing thedimensions of the dielectric resonator 12 but by placing a seconddielectric tuning element 44 in proximity to the dielectric resonator12. In this example of FIG. 14, the tuning element 44 is a cylindricalannulus larger than the outer diameter of the dielectric resonator 12and aligned with axis 14. The tuning element 44 is attached to amechanism 46 (for example, a rack and pinion lead screw or the like)allowing it to be moved along the axis as indicated by movement arrow 50to change the inductive coupling between tuning element 44 anddielectric resonator 12 thereby changing the resonant frequency ofdielectric resonator 12. Because tuning element 44 may fit arounddielectric resonator 12 close coupling may be established for sensitivetuning. The movement of the tuning elements 44 may be manual orautomatic according to feedback control, for example, according to senseimpedance as described above.

Referring now to FIG. 13, in an alternative embodiment, two identicaldielectric resonators 12 a and 12 b may be used with dielectricresonator 12 b acting as tuning element 44. The use of two identicalcomponents provide greatly increased tuning range and an extended regionof uniform electrical field. One or both of the dielectric resonator 12a and dielectric resonator 12 b may provide for electrical fieldsgenerating plasma, by which it is meant that the desired plasma may beformed inside one of the rings only, or inside both rings, dependingupon the gas flow conditions, the geometry of the torch, the location ofan ignition source and the selected resonant mode.

Alternatively, in either of the above examples depicted in FIG. 13 andFIG. 14, the tuning elements 44 may be a metal such as aluminum, copper,or silverplated copper to provide similar tuning effects.

The relative position of tuning element 44 with respect to thedielectric ring antenna alters the resonant frequency. The resonantfrequency can be expressed as a function of the coupling coefficient kbetween the dielectric ring and the tuning element 44. Couplingcoefficient k is a number between 0 and 1. In the absence of the RFshield, qualitatively, k increases as the tuning element is broughtcloser to the dielectric ring. The formulas below are for qualitativeanalysis only—a better estimate of the resonant frequency can beobtained by computer simulation of electromagnetic fields, such as byusing ANSYS-HFSS software.

The general expression for the resonant frequency of two coupledresonators is given by:

$\begin{matrix}{f_{a,b} = \frac{f_{1}^{2} + {f_{2}^{2} \pm \sqrt{\left( {f_{1}^{2} - f_{2}^{2}} \right)^{2} + {4k^{2}f_{1}^{2}f_{2}^{2}}}}}{2\left( {1 - k^{2}} \right)}} & (3)\end{matrix}$

where:

-   -   f_(a),f_(b)—resonant frequencies of the parallel and        anti-parallel modes    -   k—coupling coefficient (0<k<1)    -   f₁—resonant frequency of the dielectric ring    -   f₂—resonant frequency of the tuning element (f₂=0 if metal)

There are 2 cases of special interest:

-   -   1. For a tuning element 44 made of metal, whether the same size        as the dielectric ring or not (such as depicted in both FIGS. 13        and 14,) the expression above simplifies to:

$f = {\frac{f_{1}}{\sqrt{1 - k^{2}}}.}$

-   -   2. For two identical rings, as in FIG. 13, where the rings are        both dielectric, there are two possible modes of operation,        depending on the operating frequency. In a lower frequency mode,        the polarization currents in the two rings flow in the same        direction about the axis, i.e., they are parallel or in phase.        The frequency of this mode is approximately given by

$f_{a} = {\frac{f_{1}}{\sqrt{1 + k}}.}$

-   -    Alternatively, in the case of a higher frequency mode, the        polarization currents in the two rings flow in opposite        directions about the axis, i.e., they are anti-parallel or 180        degrees out of phase. The frequency of the second mode is        approximately given by

$f_{b} = {\frac{f_{1}}{\sqrt{1 - k}}.}$

The two frequency modes have different field distributions. The lowerfrequency mode is the strongest in the space between the rings, whilethe higher frequency mode is strongest inside the rings and zero at themid-point between the rings.

Referring also to FIGS. 2 and 11, in this example, the coupling antenna16 may be a single loop 20 terminating a coaxial cable 22 leading to thepower supply 18 and having an axis 24 generally parallel to axis 14 tocouple electrical power inductively between the loop 20 and thedielectric resonator 12 with magnetic flux lines 26 shown in FIG. 1. Thesingle loop 20 may be adjusted as indicated by rotation arrow 43 in FIG.11 to control the degree of coupling and to provide proper alignmentwith axis 14. The result is a polarization current flow 27 within thedielectric resonator 12 (shown in FIG. 2) oscillating circumferentiallyabout axis 14 at the resonant frequency of the dielectric resonator 12.

Referring now to FIG. 3, the electric field 28 within the dielectricresonator 12 at a given instant in time is substantially tangential tothe inner and outer circumferential peripheries of the dielectricresonator 12 representing a purely inductive field where parasiticcapacitive coupling has been substantially eliminated. The electricfield 28 is believed to be of such a high quality because the dielectricresonator is at once its own capacitor and an inductor and thereforeelectric-potential is exactly zero everywhere inside the dielectricresonator 12 and in the space around the dielectric resonator 12.

Referring again to FIG. 1, a gas source 32, for example, argon for anargon-based plasma may be provided through a regulator 34 to a gas port36 directing gas along axis 14 through the center of the dielectricresonator 12. Within the dielectric resonator 12, the high electricalfields convert the gas to plasma 40 that may flow along axis 14. Thedistance of flow is determined by the lifetime of the plasma excitation.Free electrons can always be found in a gas due to naturally occurringbackground ionizing radiation. When the gas is placed in a region ofhigh intensity electric field the electrons are accelerated and collidewith neutral molecules, producing additional electrons by ionization. Ifthe electric field is sufficiently strong, the number of ionizationsincreases exponentially leading to a process known as electron avalancheand the formation of plasma. In low pressure gas, plasma is principallysustained by the continued acceleration of electrons by the electricfield and ionizing collisions with the neutrals. In thermal plasma atatmospheric pressure, the flow of current through the plasma heats thegas to very high temperature which also helps to sustain the plasma.

The dielectric resonator 12 may be placed in a radiofrequency shield 42to reduce power loss due to radiation of electromagnetic energy,minimize human exposure to high intensity nonionizing radiation andcontrol electromagnetic interference. The shield 42 may be connected tothe return of the coaxial cable 22.

The use of the dielectric resonator 12 instead of a conductive metallicmulti or single loop coil directly driven by an amplifier providesmultiple benefits including:

Energy losses in the dielectric resonator 12 are one to two orders ofmagnitude lower than the conduction losses in a conventional coil. Inmany applications, this may completely eliminate the need for fluidcooling, greatly reducing the size, cost, and complexity of the plasmasource. In semiconductor processing applications, it may be possible toeliminate the need for environmentally damaging dielectric coolingfluids.

The extremely low energy losses in the dielectric resonator 12 translateinto a very large electric field strength during the plasma ignitionphase, when no power is absorbed by the plasma. This makes for easierand more reliable ignition of the plasma discharge.

The self-resonant nature of a dielectric resonator 12 greatly simplifiesor eliminates the need for an external impedance matching networkbetween the dielectric resonator 12 and the power supply 18, thusreducing the size, cost, and the complexity of the plasma source.

The use of ceramic materials, such as alumina, in the dielectricresonator 12 provides a plasma generator compatible withultra-high-vacuum processes that can be placed directly inside a vacuumchamber in order to improve the coupling to the plasma or to accommodatelimited space available for the plasma source.

Creating the dielectric resonator 12 from ceramic materials, such asalumina which have high thermal conductivity, allows for rapid heatremoval by conduction. If the dielectric resonator 12 is in directcontact with plasma, this can enable an efficient cooling of the plasmagas, a particularly important feature in gas-discharge laserapplications.

The use of ceramic materials, such as alumina for the dielectricresonator maintains good mechanical and electric characteristics atextremely high temperatures in excess of 1,000 degrees Centigrade, whichmakes a dielectric resonator 12 well suited to applications involvinghigh-temperature atmospheric plasma.

Pure inductive field, extremely low losses, high-temperature operation,and high thermal conductivity, possible with the present design, allenable operation at power levels well in excess of what is possibletoday with the conventional inductively coupled plasma technology. Themaximum power limit will depend on the size of the dielectric resonator,the cooling provided, and the electric breakdown in the RF shield andcoupling structures. It is estimated that a 2″ OD ring could operate at2 kW power level when cooled by natural convection alone, 10 kW withforced air cooling, and 100 kW with water cooling. Much greater powerlevels may be realised with a large ICR heating antenna which couldoperate at tens of MW.

Referring now to FIG. 4, in an alternative configuration dielectricresonator 12 may provide for radially extending standoffs 52 that may,for example, support the dielectric resonator 12 against a supportingstructure such as a tubular shield 42 shown in FIG. 1. The ends of thestandoffs 54 may be plated with a metal in order to reduce thermalresistance to a metal enclosure to assist in cooling of the dielectricresonator 12 which may also be cooled by natural convection or forcedflow of air around the standoffs 52.

Referring now to FIG. 5, particularly for larger dielectric resonators12, the dielectric resonator 12 may be assembled from multiple annularsectors 58 placed together at seams 60 being an abutment of metal platedend surfaces 62. The small amount of non-dielectric material does notsignificantly impact the benefits of the dielectric.

Referring now to FIG. 6, the dielectric resonator 12 may be constructedout of multiple thin rings 64 aligned along common axis 14 held apart bythin insulating spacers

Smaller rings may be easier to manufacture and transport and the gapsbetween the end surfaces 62 may provide improved cooling whilepreventing undesirable flow of dielectric polarization currents in theaxial direction.

Referring now to FIG. 7, a similar result may be achieved by fabricatingthe dielectric resonator 12 in the form of an elongated tube 68 having acentral axial bore 70 and outer circumferential notches 72 serving toprevent axial polarization currents.

Referring now to FIG. 8, it will be appreciated that the dielectricresonator 12 need not be a ring but that a toroidal plasma 40 may begenerated around the outer periphery of a dielectric resonator 12 in theform of a disk 74. The toroid of the plasma 40 may be centered aboutaxis 14 being an axis of symmetry of the disk 74. Proper selection ofthe resonant mode ensures a primary circumferential current component 27in the resonance of the disk 74.

Referring now to FIG. 9, by establishing a series of circular steps 76of increasing height as one moves toward the center of the disk 74, theplasma 40 may be displaced to an opposite face of the disk 74 of thedielectric resonator 12. The idea behind the steps 76 is to address thefact that in a simple ring or disk, the electric field is zero on theaxis and increases nearly linearly towards the outer radius. The fieldand the plasma are most intense near the ring. The steps serve toincrease the polarization current at smaller radii (by increasing thetotal thickness of the ring) so that the induced electric field is moreuniform between the axis and the outer radius. It is believed that thismay improve radial plasma uniformity. As far as displacing the plasma isconcerned, plasma on the other side of the disk would have to besuppressed by high-vacuum or higher gas pressure, for example.

Referring now to FIG. 10, in one embodiment the dielectric resonator 12may provide for a convergent-divergent nozzle 111 for the purpose ofaccelerating hot subsonic plasma flow 80 into supersonic plasma flow 82,in applications such as plasma cutting and welding or rocket engines Inthis case, the dielectric resonator 12 includes a central bore 70 thatnecks inward to a smaller diameter 84, for example, to produce a deLaval nozzle downstream from the point of plasma generation.

It will be appreciated that that many variants shown in the above Figs.may be combined in various ways. For example, the standoffs 52 of FIG. 4can be combined with the rocket nozzle of FIG. 10 in order to facilitateheat removal, or the notches 72 shown in FIG. 7 can be implemented inthe disks of FIGS. 8 and 9, in the form of circumferential grooves cutdownward into one of the faces of the disk 74 to promote the desiredcurrent flow patterns.

Referring now to FIG. 12, other methods of exciting the dielectricresonator 12 into resonance may be employed, for example, placing thedielectric resonator 12 at the end of a waveguide 89 directed generallyperpendicular to the axis 14 driven by a microwave source. An opening 90of the waveguide 89 may be controlled by an iris mechanism that may openand close a pair of irises 94 as indicated by arrows 92 to control thedegree of coupling between the microwave source and the dielectricresonator 12.

The present invention may be used in an optical emission spectrometer(OES) where their purpose is to excite the atomic and molecular speciesin an unknown chemical sample and produce light. The spectroscopicanalysis of the light emitted by the plasma is used to determine thetype and quantity of the chemical substance present in the sample. Thepresent invention may also be used in a mass spectrometer (MS) where thepurpose is to create ions of a sample material introduced into theplasma. The ions are extracted from the plasma and are transported intoa vacuum system and are mass analysed. Plasma properties criticallyaffect the analytical performance of an OES, in terms of the ability toprocess samples in aqueous or organic solvents without extinguishing theplasma, the ability to operate on different plasma gases for improvedsafety and economy, the ability to detect different kinds of chemicals,the ability to accurately measure a very large range of analyteconcentrations, the ability to detect extremely small concentrations ofthe analyte, the ability to process many samples in a short amount oftime, the ability to produce stable results when measurements arerepeated over a long period of time, etc. Plasma properties criticallyaffect the analytical performance of a MS in a similar way as theyaffect the performance of an optical-emission spectrometer. Unique toMS, the ions created in an atmospheric pressure plasma must betransferred to a high-vacuum environment of the mass-spectrometerthrough the, so called, interface part of the MS. The interface containsmultiple metallic cones with small orifices which separate the regionsof different pressure. The cone whose one side is in direct contact withatmospheric pressure plasma is known as the sampler cone. Theperformance of the sampler cone is most critically affected by theparasitic capacitive coupling of a conventional RF coil, leading toreduced ion transmission, arcing, and erosion of the cone. Most commonlyused inductively coupled plasma sources for MS operate at radiofrequencies up to 40 MHz.

The plasma source may also be used as an atomisation source for atomicabsorption (AA) spectroscopy.

Typical plasma sources for this application may operate atradio-frequencies up above 40 MHz with much higher frequenciesimplemented by this design (i.e. the present invention). Alternatively,the design may provide plasma at microwave frequencies, such as 915 MHzor 2,450 MHz, using a magnetron device as a source of large amount ofmicrowave power.

Existing designs for microwave plasma generators are dominated bycapacitive coupling or retain a significant amount of parasiticcapacitive coupling, which has a serious negative impact on the plasmasource, or have form factors that would require significantmodifications to the conventional mechanical, optical, and chemicalinterface to the rest of the spectrometer, an interface which has provenitself over many years of operation of radio frequency OES in the field(i.e. as proven with ICP plasma generation systems). The parasiticcapacitive coupling present in prior art microwave plasma generatorssuch as Surfatron, Beenakker cavity, Okamoto cavity, Surfaguide,Multi-helix torch, TIA torch, etc. has a serious negative impact on theperformance of an inductive plasma source leading to: a) plasmanon-uniformities, b) poor control over ion speeds and trajectories, c)deposition or sputtering of the walls of the plasma chamber, d) powerdissipation in non-essential plasma processes, and e) limitation on theamount of electrical power that can be efficiently coupled into usefulplasma processes.

In contrast, the plasma source of the present design may extend theoperation of the conventional radio-frequency inductively coupled plasmasources to microwave frequencies, practically eliminating parasiticcapacitive coupling which has limited previous designs, while requiringminimum modifications to the established mechanical, optical, andchemical interface with the rest of the spectrometer. In addition, theextremely low losses of the novel field applicator, allow for a completeelimination of the fluid cooling system, thus reducing the size, cost,and the complexity of the spectrometer and improving reliability. Theplasma source of the present design also allows a range of differentplasma gases to be used including gases comprising nitrogen or air. Inone preferred embodiment the plasma is sustained in air. In anotherpreferred embodiment the plasma is sustained in nitrogen.

Referring now to FIG. 15 a microwave inductively coupled plasma sourcefor optical-emission spectroscopy 102, which uses a dielectric resonator12 of the present invention, made out of high-density Alumina (Al₂O₃)ceramics in the form of a circular annulus. The dielectric resonator 12may be supported within cylindrical radio-frequency shield 42 made ofmetal, such as aluminum, and has several circular openings 104, 106, and108 each surrounded with aluminum tubular extensions 110, 112, and 114respectively. The tubular extensions 110-114, are designed to have asufficiently small diameter and sufficiently long length to formcylindrical waveguides below cutoff, greatly attenuating the propagationof microwaves through the extension tubes, as is well understood in themicrowave art, in order to minimize the leakage of microwave energyoutside of the shield 42.

Microwave power 118 from waveguide 89 communicating with magnetron 120is provided at a frequency of 2,450 MHz a n d applied to the dielectricresonator 12 through a rectangular opening 122 in the shield 42 by themeans of a coupler 124. The resonant frequency of the dielectricresonator 12 can be finely adjusted by varying the axial location of thetuning element 44, made in the form of an aluminum ring, positionedcoaxially with the ring of dielectric resonator 12.

A triaxial manifold 125 is directed along the axis 14 centered withinopening 104 and aligned with inner diameter of dielectric resonator 12and made out of quartz or alumina tubing. The triaxial manifold is inthe form of a conventional torch which may be similar to that used withinductively coupled plasmas. A plasma cooling gas 126 is applied to anouter ring of the triaxial manifold 125 while a plasma auxiliary gas 128is applied to the next inner ring and the center bore receives thedissolved analytical sample or solid particles of sample 130 from asample source 132 to be analyzed. The sample 130 is in the form of anaerosol, or discrete particles, entrained in a gas, that may be directlyintroduced into the plasma 40.

Light 134 emitted from the plasma 40 in a direction radial to axis 14passes through the tubular extension 112 for analysis by a light sensor136 coupled to an analyzing computer 138 that may determine frequencycomponents of the light 134 according to methods known in the art.Alternatively or in parallel, for the purposes of the, so called, axialOES, light 140, emitted by the plasma 40 in the axial direction of axis14, is transferred through the tubular extension 110 for furtherspectroscopic analysis by a similar light sensor 136 (not shown forclarity). The tubular extension 110 also directs the hot plasma gasesand chemical products 142 to an exhaust venting system (not shown.) Theopening 108 and the tubular extension 114 allow for air cooling of theplasma generator 12 by natural convection or by forced flow of air.

The optical emission spectrometer of the present invention preferablycomprises a plasma generator, the plasma generator comprising adielectric resonator, a dispersive element for dispersing light emittedby the plasma according to the wavelength of the light, and an opticaldetector for detecting the dispersed light.

FIG. 16 is a simplified schematic cross-sectional view of a massspectrometer incorporating the dielectric resonator of the presentinvention. Most commonly used inductively coupled plasma sources for MSoperate at radio frequencies up to 40 MHz. Several designs have beenproposed and tested with a goal of extending the operation of the plasmasources for MS to microwave frequencies, such as 915 MHz or 2,450 MHz,where a magnetron device could serve as an efficient source of largeamount of microwave power. The existing analytical results indicate thatmicrowave excited plasma offers unique advantages that complement theanalytical power of a radio-frequency based plasma sources. However, oneof the key obstacles in the ability to produce a high quality analyticalplasma at microwave frequencies has been the lack of a field applicatorcapable of producing a pure inductive coupling to the plasma. All of thedesigns proposed to date are either dominated by capacitive coupling orretain a significant amount of parasitic capacitive coupling, which hasa serious negative impact on the plasma source performance as previouslyoutlined. In addition, all of the previous designs require significantmodifications to the conventional mechanical, optical, and chemicalinterface to the rest of the spectrometer, an interface which has provenitself over many years of operation of radio-frequency MS in the field.

In contrast, the plasma source for MS, based on the field applicatoraccording to the present invention, extends the operation of theconventional radio-frequency inductively coupled plasma sources tomicrowave frequencies, practically eliminating parasitic capacitivecoupling which has limited previous designs, while requiring minimummodifications to the established mechanical, ion, and chemical interfacewith the rest of the spectrometer. In addition, the extremely low lossesof the novel field applicator, allow for a complete elimination of thefluid cooling system, thus reducing the size, cost, and the complexityof the spectrometer.

FIG. 16 shows a schematic simplified cross-section of a microwaveinductively coupled plasma source for mass spectrometry 200, which usesa field applicator 12 of the present invention, made out of high-densityAlumina (Al₂0₃) ceramic in the form of a ring. The microwave inductivelycoupled plasma source for MS 200 has many components in common with themicrowave inductively coupled plasma source for OES 102 shown in FIG.15, and like components have the same identifiers. Additional componentsshown in FIG. 16 will now be described. The sampler cone 201 has a smallorifice 202 and the skimmer cone 203 has a small orifice 204. The regionbetween the sample cone 201 and the skimmer cone 203 is maintained at alow pressure by exhausting the gas 205 by means of a vacuum pump (notshown). The ionized sample 206 enters the low pressure region betweenthe sample and skimmer cones through the orifice 202. Ions 207 arefurther transmitted through the orifice 204 into the high-vacuum regionof the mass-spectrometer. The mass spectrometer comprises ion focusingcomponents 209 which comprise at least one ion focusing element, a massanalyser 210 and an ion detector 211. There may be two or more stages ofpumping (not shown) disposed within the mass spectrometer. The massspectrometer is controlled by a controller (not shown), which ispreferably a computer. Detected signal from ion detector 211 isrecorded, preferably also using a computer, which may be the samecomputer as is used as the controller. The heated plasma gas 208 whichhas not penetrated the orifice 202 is exhausted through the annularregion between the RF shield 42 and the sample cone 201.

Preferably the optical emission spectrometer or the mass spectrometercomprises a plasma generator according to the present invention whereinthe radiofrequency power source provides between 0.5 and 2 kW of powerinto the plasma.

The performance of an optical emission spectrometer according to thepresent invention was compared with that of a conventional ICP opticalemission spectrometer operating in radial viewing mode. A conventionalICP torch was located within the central aperture of the dielectricfield applicator, the torch being connected to the gas supplies of thespectrometer. The dielectric field applicator and torch were mountedsuch that the plasma formed within the central aperture of thedielectric field applicator was aligned for viewing by a high-resolutionEchelle spectrometer in radial viewing mode. Advantageously the plasmagenerator was operated with both air and nitrogen without any change tothe plasma generator system due to the unique way in which the ceramicring works as both an inductor and a tuning device and because theelectrical coupling into the plasma is substantially purely inductivewith negligible capacitive coupling.

FIG. 17 shows a plot of signal intensity in counts per second (IR) vs.element concentration for a range of elements utilizing a range of hardand soft lines measured using an optical emission spectrometer of thepresent invention. The energy sums for the five lines are: Ca3968, 9.23eV (3.12 eV energy of excitation and 6.11 eV energy of ionization);Cu2165, 5.73 eV (excitation energy); Cu3247, 3.82 eV (excitationenergy); Mg2802, 12.07 eV (4.42 eV energy of excitation and 7.65 eVenergy of ionization); Mn2794, 12.25 eV (4.82 eV energy of excitationand 7.42 eV energy of ionization).

Linearity was also examined for a solution containing 3% salt matrix.The results obtained are shown in FIG. 18 which indicates that linearityis maintained despite the presence of a large concentration of sodiumfrom the salt, which being more easily ionized can modify thedistribution of detected ionic and atomic lines and line emissionlevels.

FIG. 19 (a) to (d) are peak profile plots showing measured peakintensities from a multielement standard and baselines which arebackground signals from measured blanks (deionised water), for aconventional argon ICP source and the plasma source of the presentinvention operating with air. The multielement standard contained 0.2ppm Ba and Mg, 1 ppm Cu, 5 ppm Ni. Cu and Ni are soft atom lines andgive much the same performance with conventional argon ICP and the airplasma source of the present invention. Ba is a harder ion line andperforms better in the conventional argon ICP plasma, but the peakintensity in the air plasma is only a little less than half that in theargon ICP plasma. Other forms of dielectric resonator are contemplated,two examples of which are presented in FIGS. 20 and 21.

FIG. 20 is a perspective partial cutaway view of a dielectric resonatorwhich is in the form of a ceramic ring 12 together with an RF shield 42in direct contact with an outer surface of the dielectric resonator 12.This configuration offers the advantage of smaller size and bettertransfer of heat to the RF shield 42. The surface of the ceramic ring 12which is in contact with the RF shield 42 may be plated with metal.

FIG. 21 is a perspective partial cutaway view of a dielectric resonatorin the form of two coaxial ceramic rings 12 c and 12 d, together withtwo concentric RF shields. The outer surface of the larger ring 12 c isin direct contact with outer RF shield 42 aa. The inner surface of thesmaller ring 12 d is in direct contact with inner RF shield 42 b. Theplasma 40 may be formed in the annular gap between the rings 12 c and 12d.

Certain terminology is used herein for purposes of reference only, andthus is not intended to be limiting. For example, terms such as “upper”,“lower”, “above”, and “below” refer to directions in the drawings towhich reference is made. Terms such as “front”, “back”, “rear”, “bottom”and “side”, describe the orientation of portions of the component withina consistent but arbitrary frame of reference which is made clear byreference to the text and the associated drawings describing thecomponent under discussion. Such terminology may include the wordsspecifically mentioned above, derivatives thereof, and words of similarimport. Similarly, the terms “first”, “second” and other such numericalterms referring to structures do not imply a sequence or order unlessclearly indicated by the context.

When introducing elements or features of the present disclosure and theexemplary embodiments, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of such elements orfeatures. The terms “comprising”, “including” and “having” are intendedto be inclusive and mean that there may be additional elements orfeatures other than those specifically noted. It is further to beunderstood that the method steps, processes, and operations describedherein are not to be construed as necessarily requiring theirperformance in the particular order discussed or illustrated, unlessspecifically identified as an order of performance. It is also to beunderstood that additional or alternative steps may be employed.

The term “ring” should be understood to generally mean a topologicalsurface of genius one and not require nor exclude, for example, acircular profile, radial symmetry or particular aspect ratios of with adiameter to height except as explicitly noted.

It is specifically intended that the present invention not be limited tothe embodiments and illustrations contained herein and the claims shouldbe understood to include modified forms of those embodiments includingportions of the embodiments and combinations of elements of differentembodiments as come within the scope of the following claims. All of thepublications described herein, including patents and non patentpublications are hereby incorporated herein by reference in theirentireties.

We claim:
 1. A method of analyzing a substance comprising the steps of:generating plasma using a plasma generator including a dielectricresonator structure and a radiofrequency power source electricallycoupled to the dielectric resonator structure to promote an alternatingpolarization current flow at a natural resonant frequency of thedielectric resonator structure about the axis to generate plasma in anadjacent gas, introducing a gas into a region adjacent to the dielectricresonator structure; exciting the dielectric resonator structure at anatural resonant frequency to generate plasma in the introduced gas;introducing substance to be analyzed into the plasma; dispersing lightemitted by the substance according to the wavelengths of the light orseparating ions of the substance created by the plasma according totheir mass to charge ratio; detecting either light emitted by thesubstance according to the wavelengths of the light or ions of thesubstance created by the plasma according to their mass to charge ratio;and determining the elemental composition of the substance either fromthe wavelengths of light detected or from the mass to charge ratio ofthe ions detected.
 2. The method of claim 1 wherein in step (c) theplasma is generated in the introduced gas by an electric field whereinthe electric field is substantially only coupled by induction, therebeing negligible capacitive coupling.
 3. The method of claim 2 whereinthe introduced gas comprises nitrogen or air.
 4. The method of claim 1wherein the introduced gas comprises nitrogen or air.
 5. The method ofclaim 1 wherein in the radiofrequency power source automatically seeksthe natural resonant frequency of the dielectric resonator structure tooutput radiofrequency power at or substantially at the natural resonantfrequency of the dielectric resonator structure.
 6. The method of claim2 wherein the radiofrequency power source automatically seeks thenatural resonant frequency of the dielectric resonator structure tooutput radiofrequency power at or substantially at the natural resonantfrequency of the dielectric resonator structure.
 7. The method of claim3 wherein in the radiofrequency power source automatically seeks thenatural resonant frequency of the dielectric resonator structure tooutput radiofrequency power at or substantially at the natural resonantfrequency of the dielectric resonator structure.
 8. The method of claim4 wherein in the radiofrequency power source automatically seeks thenatural resonant frequency of the dielectric resonator structure tooutput radiofrequency power at or substantially at the natural resonantfrequency of the dielectric resonator structure.
 9. The method of claim1 wherein the dielectric resonator structure provides a dielectricmaterial extending around a central axis providing a continuous circularpath within the dielectric material of the dielectric resonator.
 10. Themethod of claim 1 wherein the radiofrequency power source electricallycouples to the dielectric resonator structure to promote an alternatingpolarization current flow at a natural resonant frequency of thedielectric resonator structure and directed circumferentially about acentral axis along a continuous circular path within the dielectricmaterial of the dielectric resonator to generate plasma in the gas tointeract with the material to be studied.
 11. The method of claim 1wherein the dielectric resonator structure provides a dielectricmaterial extending around a central axis and wherein the radiofrequencypower source electrically couples to the dielectric resonator structureto promote an alternating polarization current flow at a naturalresonant frequency of the dielectric resonator structure and directedcircumferentially about the central axis along a continuous circularpath within the dielectric material of the dielectric resonator togenerate plasma in the gas to interact with the material to be studied.12. The spectrometer of claim 1 wherein the dielectric resonatorstructure is electrically coupled to the plasma substantially only byinduction, there being negligible capacitive coupling.
 13. Thespectrometer of claim 1 wherein the dielectric resonator has a qualityfactor of greater than 100 and an electrical resistivity greater than1×10¹⁰ Ω·cm.
 14. The spectrometer of claim 1 wherein the dielectricresonator has a dielectric constant with a loss tangent of less than0.01.
 15. The spectrometer of claim 1 wherein the dielectric resonatorhas a dielectric constant of greater than five.
 16. The spectrometer ofclaim 1 wherein a dielectric material of the dielectric resonator isselected from the group consisting of alumina (Al₂O₃) and calciumtitanate (CaTiO₃).
 17. The spectrometer of claim 1 wherein thedielectric resonator is selected from the group consisting of a ring anda cylindrical annulus having a central opening along the axis.
 18. Thespectrometer of claim 1 wherein the dielectric resonator has a centralopening of at least one millimeter in diameter.
 19. The spectrometer ofclaim 1 wherein the dielectric resonator has a central opening which iscircular and has a diameter of between 15 mm and 25 mm.
 20. Thespectrometer of claim 1 wherein the dielectric resonator and theradiofrequency power source are placed within a waveguide and theradiofrequency power source is electrically coupled to the dielectricresonator structure by radiation through the waveguide.